Actuator and transducer

ABSTRACT

An actuator comprises a magnet yoke ( 10   a   , 10   b ) and a carrier member ( 2 ) movable relative to the magnet yoke. The magnet yoke ( 10   a   , 10   b ) has at least one permanent magnet ( 14 ) and the carrier member ( 20 ) is positioned in the magnetic field produced by this magnet. The carrier member ( 20 ) has an auxiliary magnetic member ( 28 ) that produces a relative bias force between the carrier member ( 20 ) and the magnet yoke ( 10   a   , 10   b ). The bias force will be used to compensate for a weight applied to the device and acts as a bearing with a very large compliance. The carrier member ( 2 ) also comprises a coil ( 26 ). Passing current through the coil ( 26 ) produces a Lorentz force for further control of the actuator; alternatively, the device provides a velocity transducer by sensing the EMF generated in the coil by relative motion of the carrier member ( 20 ) and magnet yoke ( 10   a   , 10   b ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention also relates to a lithographic projection apparatus, andmore particularly to a lithographic projection apparatus that has aLorentz actuator connected to a mask table or a substrate table of thelithographic projection apparatus.

The invention relates to actuators, such as Lorentz actuators, and alsoto velocity transducers.

2. Discussion of Related Art

Lorentz actuators comprise a permanent magnet, which produces a magneticfield, and a current element positioned in the magnetic field. They workon the same principle as an electric motor, namely that charge carriersmoving through a magnetic field experience a force mutuallyperpendicular to their velocity and the magnetic field, known as theLorentz force. The force is given by J×B, where J is the current vectorresulting from the velocity of the charge carriers and B is the magneticfield vector. This Lorentz force is used to induce motion or to providea bias force between the moving parts of the actuator.

Lithographic projection apparatuses can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the mask(reticle) may contain a circuit pattern corresponding to an individuallayer of the IC, and this pattern can then be imaged onto a target area(die) on a substrate (silicon wafer) which has been coated with a layerof photosensitive material (resist). In general, a single wafer willcontain a whole network of adjacent dies which are successivelyirradiated through the reticle, one at a time. In one type oflithographic projection apparatus, each die is irradiated by exposingthe entire reticle pattern onto the die in one go; such an apparatus iscommonly referred to as a waferstepper. In an alternative apparatus,each die is irradiated by progressively scanning the projection beamover the reticle pattern, and thus scanning a corresponding image ontothe die; such an apparatus is referred to as a step-and-scan apparatus.Both of these types of apparatus require highly accurate relativepositioning of the mask and substrate tables, which is generallyaccomplished with the aid of at least one Lorentz actuator. Moreinformation with regard to these devices can be gleaned fromInternational Patent Application WO 97/33204.

Up to very recently, apparatus of this type contained a single masktable and a single substrate table. However, machines are now becomingavailable in which there are at least two independently movablesubstrate tables; see, for example, the multi-stage apparatus describedin International Patent Applications WO 98/28665 and WO 98/40791. Thebasic operating principle behind such multi-stage apparatus is that,while a first substrate table is underneath the projection system so asto allow exposure of a first substrate located on that table, a secondsubstrate table can run to a loading position, discharge an exposedsubstrate, pick up a new substrate, perform some initial alignmentmeasurements on the new substrate, and then stand by to transfer thisnew substrate to the exposure position underneath the projection systemas soon as exposure of the first substrate is completed, whence thecycle repeats itself; in this manner, it is possible to achieve asubstantially increased machine throughput, which in turn improves thecost of ownership of the machine

In currently available lithographic devices, the employed radiation isgenerally ultra-violet (UV) light, which can be derived from an excimerlaser or mercury lamp, for example; many such devices use UV lighthaving a wavelength of 365 nm or 248 nm. However, the rapidly developingelectronics industry continually demands lithographic devices which canachieve ever-higher resolutions, and this is forcing the industry towardeven shorter-wavelength radiation, particularly UV light with awavelength of 193 nm or 157 nm. Beyond this point there are severalpossible scenarios, including the use of extreme UV light (EUV:wavelength˜50 nm and less, e.g. 13.4 nm or 1 m), X-rays, ion beams orelectron beams.

One problem with Lorentz actuators is that, when no current flows, thereis no force between the moving parts. When a current is caused to flowto overcome this, it results in dissipation of heat in the device. Thisis particularly a problem in applications which require the actuator todeliver a bias force, e.g. to support the weight of a component undergravity. With this continuous need to compensate for weight, a basepower dissipation is unavoidable, and can cause problems with heatsensitive apparatus, such as optical devices which require accuratealignment; on the other hand, it necessitates the provision ofadditional cooling power.

Another problem is that, when such actuators support a load in order toact as isolation bearings, the stiffness of the bearing should be low soas to avoid the transmission of vibrations. Conventionally, it has beendifficult to provide such low-stiffness isolation bearings.

Velocity transducers can also operate on the Lorentz principle, byvirtue of the fact that the motion of a component through a magneticfield induces a current flow or a resulting EMF which can be measured.In order to measure velocities down to very low frequencies, it isnecessary to have a transducer with a very low frequency of resonance,which conventionally has been difficult to achieve. This is because ofthe problems in producing a transducer with a very low stiffness.

SUMMARY OF THE INVENTION

It is an object of the present invention to alleviate, at leastpartially, some of the above problems.

Accordingly, the present invention provides a device comprising:

a first member comprising at least one main magnet, and

a second member comprising at least one current element for carrying anelectric current, for electromagnetic interaction with said main magnet,

characterized in that said second member further comprises an auxiliarymagnetic member which interacts with the magnetic field of said mainmagnet to produce a bias force between said first and second members.

The invention also relates to a lithographic projection apparatuscomprising a radiation system for supplying a projection beam ofradiation; a mask table provided with a mask holder for holding a mask;a substrate table provided with a substrate holder for holding asubstrate; a projection system for imaging an irradiated portion of themask onto a target portion of the substrate; and further comprising aLorentz actuator connected to at least one of the mask table and thesubstrate table.

The auxiliary magnetic member can be a permanent magnet. Alternatively,it can comprise a ferromagnetic material (e.g. a soft-iron member). Inthis latter case, as long as the stroke of movement of the currentelement/auxiliary magnetic member is relatively small (as will generallybe the case in applications in short-stroke lithography actuators, forexample)—such that the auxiliary magnetic member remains biased to oneside of the centerline of the whole assembly—magnetic fluxes goingthrough the ferromagnetic material of the auxiliary magnetic member willproduce a bias force component in the desired direction; while less thanthat produced in the case of a permanent magnetic material, this forcewill be quite sufficient for particular applications.

The device according to the invention can be substantially planar orcylindrical, and the main magnet. can be magnetized perpendicular orparallel to the bias force.

Preferably, the device further comprises a third member, also comprisingat least one further main magnet.

The current element may be a coil, and the auxiliary magnetic member ispreferably located at a plane substantially centrally between two halvesof the coil.

Advantageously, the effective stiffness of the device is 200 N/m or lessin magnitude, and ideally close to zero.

The device can be used as an actuator and/or a velocity transducer.

The device can have two second members stiffly connected to each otherand arranged such that opposite parasitic torques are generated in eachsecond member, which thereby cancel out.

Advantageously, the actuator and/or transducer of the present inventioncan be used in a lithographic projection apparatus. A great advantage ofthe invention in such an application is that it provides a bias forcecapable of counteracting, for example, the weight of the table (chuck)in a wafer stage or reticle stage, and yet does so without the heatdissipation associated with current flow, thus helping to maintain awell-defined and constant local temperature. This is important, sincethe nanometer-accuracy commonly required of such apparatuses can only besatisfactorily achieved in a highly controlled environment, whereinunnecessary sources of heat and/or contamination (e.g. as a result ofevaporation or outgassing) are highly undesirable. Such considerationsare of particular importance in a vacuum environment, in which contextit should be noted that lithographic apparatus for use with radiationtypes such as EUV, electron beams, ion beams, 157-nm UV, 126-nm UV, etc.will most probably comprise a vacuum along at least part of theradiation path within the apparatus.

In a manufacturing process using a lithographic projection apparatusaccording to the invention, a pattern in a mask is imaged onto asubstrate which is at least partially covered by a layer ofenergy-sensitive material (resist). Prior to this imaging step, thesubstrate may undergo various procedures, such as priming, resistcoating and a soft bake. After exposure, the substrate may be subjectedto other procedures, such as a post-exposure bake (PEB), development, ahard bake and measurement/inspection of the imaged features. This arrayof procedures is used as a basis to pattern an individual layer of adevice, e.g. an IC: Such a patterned layer may then undergo variousprocesses such as etching, ion-implantation (doping), metallization,oxidation, chemo-mechanical polishing, etc., all intended to finish offan individual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4.

Although specific reference has been made hereabove to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetarea”, respectively.

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying schematic drawings,whereby:

FIG. 1 illustrates a first embodiment of the invention in cross-section;

FIGS. 2(a) and 2(b) illustrate a second embodiment of the invention;

FIG. 3 illustrates a third embodiment of the invention;

FIGS. 4(a) and 4(b) give the dimensions of specific examples of thesecond and third embodiments of the invention;

FIGS. 5(a) to 5(g) illustrate further embodiments of the invention,which have configurations related to the second embodiment, togetherwith graphs of calculated performance;

FIGS. 6(a) and 6(b) illustrate yet further embodiments of the invention,which have configurations of the same type as the third embodiment,together with graphs illustrating their calculated performance;

FIGS. 7-14 depict tabulated data as elucidated further hereafter;

FIG. 15 shows an elevation of a lithographic projection apparatusemploying an actuator in accordance with the invention.

In the Figures, corresponding parts are denoted using correspondingreference symbols.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

FIG. 1 shows the configuration of a linear Lorentz actuator according toa first embodiment of the present invention. It comprises a magnet yokehaving upper and lower members 10 a and 10 b. Each member comprises aback-iron 12 which supports main magnets 14. Each main magnet is apermanent magnet which may include, for example, a ferromagneticmaterial. The direction of magnetization of each main magnet 14 isindicated by an arrow. Each member 10 a, 10 b comprises a pair ofmagnets 14, and there are four magnets in total in the magnet yoke.

Located between the upper and lower magnet yoke members 10 a and 10 b isa carrier member 20 which comprises cover plates 22, coil carrier 24 andtwo coil halves 26. The coil halves form a complete coil circuit, andare arranged to carry current perpendicular to the plane of the figure,one half carrying current in the opposite direction to the other half.In the middle of the carrier member 20 is an auxiliary magnetic member28. In this particular case, this is also a permanent magnet, and can,for example, be made of the same material as the main magnets 14. Thedirection of magnetization of the auxiliary magnetic member 28 isindicated by its arrow.

The carrier member 20 is movable relative to the magnet yoke 10 a, 10 b.In the configuration shown in FIG. 1, the interaction between the mainmagnets 14 and the auxiliary magnetic member 28 produces a force thatbiases the carrier member 20 to the right relative to the magnet yoke.This bias force is present even when no current is flowing through thecoil halves. When a current flows through the coil circuit in one sense,an additional force (Lorentz force) to the right is produced on thecarrier member 20. When the current through the coil circuit flows inthe opposite sense, a Lorentz force to the left is produced on thecarrier member 20. The net force on the carrier member 20 depends on thebias force due to the presence of the auxiliary magnetic member and theLorentz force caused by the coil current.

When the carrier member 20 is displaced within the yoke, forces in thevertical direction and also torques (principally about an axisperpendicular to the plane of FIG. 1) are produced. These are generallyreferred to as parasitic forces and torques, because they are not usefulto the operation of the device and compensation must generally be madefor them.

A Lorentz actuator or transducer also has an effective stiffness givenby the change in force per unit displacement from a nominal operatingposition, typically the central position, for a given current level.This stiffness is also referred to as parasitic stiffness, since, forparticular applications, an actuator should ideally apply a constantforce with zero or minimal stiffness.

Finite element calculations have been performed to obtain data on theoperational performance of an actuator of the type shown in FIG. 1. Inthe accompanying tables (FIGS. 7-14), the horizontal and verticaldirections in the plane of FIG. 1 are referred to as the X and Ydirections, respectively. The direction perpendicular to the plane ofFIG. 1 is the Z direction. The device is taken to have the uniformcross-section as shown in FIG. 1, extending in the Z direction. After anon-linear calculation of the magnetic field distribution in the device,the forces and torque on the carrier 20 relative to the magnet yoke 10are calculated by integration of the Maxwell stress along a contourcontaining the carrier 20. Calculated forces and torques are per unitlength in the Z direction. A typical device as here considered has alength of 80 mm in the Z direction, and this number can be used toobtain values of the forces, torques and effective stiffness of apractical device.

Three different Examples of the device are discussed hereafter, the onlydifferences between them being the dimensions of the auxiliary magneticmember 28, whereby L denotes length in the X direction, and H denotesheight in the Y direction, both in mm. In Example 1, the dimensions ofthe auxiliary magnetic member 28 (L=10, H=8) are such that all theavailable space between the coil halves 26 (L=18, H=8) is filled, unlikethe device illustrated in FIG. 1. In Example 2, the auxiliary magneticmember 28 (L=10, H=2) is relatively thin in the Y-direction, whereas inExample 3, the auxiliary magnetic member 28 (L=2, H=8) is relativelynarrow in the X-direction. In all Examples, the main magnet 14 (L=24,H=8) was comprised of neodymium-iron-boron alloy RES275, and the backiron 12 (L=52, H=10) was comprised of standard ferromagnetic iron N041.Except where otherwise indicated, the auxiliary magnetic member 28 wasalso comprised of RES275. The current density in the coil half 26 was 10A/mm².

Example 1

The table in FIG. 7 shows the results of calculations of the horizontalforce (fx), vertical force (fy), and torque (tz) about an axis in the Zdirection, exerted on the carrier member 20 for four different testconditions, but all with the dimensions of the device as specified abovefor Example 1 (L=10, H=8). For each of these four test runs, the carriermember 20 was kept in its nominal position, centrally located withrespect to the magnet yoke. Values of fx and fy are in N and values oftz are in Nm, per mm in the Z-direction.

Runs 1 and 2 were with an RES275-grade magnet as the auxiliary magneticmember 28; run 1 was with no current flowing in the coil halves, and run2 was with a current density of 10 A/mm². Run 3 was on the basis thatthe auxiliary magnetic member 28 was degraded to aceramic-material-grade, and run 4 was a control with no auxiliarymagnetic member.

The first, third and fourth runs reveal the force in the X directionproduced by the auxiliary magnetic member 28 alone, the coil 26 alone,and the combination of auxiliary magnetic member 28 and coil 26. In thisexample, with all the available space between the coil halves filledwith an RES275-grade magnet, the bias force is very large—about fourtimes the force generated by the coil carrying 10 A/mm²—and is actuallylarger than is often required. With a ceramic-material-grade auxiliarymagnetic member, the bias force is reduced so as to be roughly equal tothe force generated by the current, but the auxiliary magnetic member islocally irreversibly demagnetized by the main magnets over about 30% ofits cross-sectional area.

Further runs were performed in which the position of the carrier member20 was varied to determine stiffnesses due to magnetic coupling from theauxiliary magnetic member 28 to the main magnets 14. Coil current waskept at 0 A/mm² in order to isolate the permanent magnet effects. Thetable in FIG. 8 shows fx, fy and tz for various X displacements (Δx) andY displacements (Δy) of the carrier member 20 (Δx and Δy in mm). Fromthese results, it is possible to calculate the differential change inforce (Δfx, Δfy) in the X and Y directions as a function of Δx and Δy,as shown in the table in FIG. 9 (Δfx, Δfy in N/m, per mm in the Zdirection).

A typical actuator has a length of 80 mm in the Z direction, which givesthe following equivalent stiffnesses for a practical device as: 5.9×10³N/m in the X direction and −1.2×10⁴ N/m in the Y direction. The signconvention used when quoting these equivalent stiffnesses of an actuatordiffers from that used in the tables of FIGS. 7-14 for the followingreasons. For a simple mechanical spring, a displacement in one directionproduces a force in the opposite direction; however it is conventionalto quote the stiffness of a spring as a positive value. Devices whichexhibit the opposite behavior to a spring, such that a displacement inone direction produces an increased force in that direction, areconventionally quoted as having a negative stiffness (although in thetables these will appear as a positive value, because the displacementand force both change in the same direction).

Example 2

The tables of FIGS. 10 and 11 correspond to those of FIGS. 8 and 9,respectively, except in that they pertain to a device having thedimensions specified above for Example 2 (L=10, H=2). The magnitudes ofthe forces are obviously smaller, because the auxiliary magnetic memberhas a smaller volume. The equivalent stiffnesses for an 80 mm actuatorare 2.4×10³ N/m in the X direction and −2.0×10⁴ N/m in the Y direction.

Example 3

The tables of FIGS. 12 and 13 correspond to those of FIGS. 8 and 9,respectively, except in that they pertain to a device having thedimensions specified above for Example 3 (L=2, H=8). For an 80 mmactuator, the equivalent stiffnesses are: 1.4×10⁴ N/m in the X directionand −2.9×10⁴ N/m in the Y direction.

From all the above results, it is apparent that the bias forces producedby the auxiliary magnetic member in Examples 2 and 3 are roughly thesame as the Lorentz forces produced by a current density of 10 A/mm²; sodepending on the direction of the current, the total force can be variedfrom approximately zero to roughly double the bias force for zerocurrent by varying the current density between −10 A/mm² and +10 A/mm²(the sign giving the direction or “sense” of the current flow). In bothExamples 2 and 3, the size of the auxiliary magnetic member isconsiderably less than the space available in the center of the coil.

The parasitic stiffness in these examples ranges from about 10³ N/m to10⁴ N/m. The parasitic torque for an 80 mm device is about 0.6 Nm for afull size auxiliary magnetic member, as in Example 1, and about 0.12 Nmfor Examples 2 and 3. The parasitic torque at maximum displacement in Ycan be compensated for—e.g. by using two carrier members that arestiffly connected to each other and which, when displaced in anoperational direction, generate opposite parasitic torques which cancelout.

Embodiment 2

FIGS. 2(a) and (b) show a second embodiment of the invention. FIG. 2(a)shows a cross-section through half of the device, which looksessentially like the device of FIG. 1 turned on its end. The device ofthis embodiment has a cylindrical configuration, which is obtained byrotating FIG. 2(a) about axis 40 to produce the device shown in FIG.2(b). In FIG. 2(b), the carrier member 20 has been omitted for clarity,but the tubular region in which it is located is indicated at 42.

As will be appreciated from FIG. 2, the main and auxiliary magneticmembers are rings, magnetized in the radial direction. These magnets areroutinely available, but are more difficult to make and magnetize thanflat magnets, particularly when using anisotropic magnetic material.

Embodiment 3

Another cylindrical configuration is contemplated, according to a thirdembodiment of the invention shown in FIG. 3. As in FIG. 2(a), only halfthe device is shown in radial cross-section, and the complete device isa rotation of this figure about axis 40. In this embodiment, a singlecylindrical main magnet 14 is provided, which is magnetized in the axialdirection and is therefore somewhat simpler to manufacture thanembodiment 2. The auxiliary magnetic member 28 is still ring-shaped,substantially the same as in the second embodiment. As shown in FIG. 3,the magnetic circuit is established using back-iron pieces 12 to formthe magnet yoke. The coil halves 26 are positioned in the gaps in themagnetic circuit.

Embodiment 4

Devices such as that shown in FIG. 2, which have radially magnetizedmain magnets, are referred to as “type I”, and devices such as thatshown in FIG. 3, with an axially magnetized main magnet, will bereferred to as “type II”. From the magnetization directions of the mainmagnets 14 and auxiliary magnetic member 28 in both FIGS. 2 and 3, itwill be apparent that the carrier member 20 will experience a bias forcein the axial direction towards the top of each figure. Thus, thesedevices are useful (when oriented with the axis in the verticaldirection) for supporting an axial load, such as bearing a load undergravity. In the type I device, the magnetization direction of the mainmagnets 14 is perpendicular to the bias force. In the type II device,the magnetization direction of the main magnets 14 is parallel to thebias force.

The planar, or linear, configuration of the device according to thefirst embodiment of the invention, shown in FIG. 1, has the advantage ofmanufacturing ease for the magnets, since they are rectangular-sidedsolids magnetized in a single direction. However, manufacture of thecoil and guidance of the coil carrier member 20 can be relativelydifficult. The cylindrical configurations of embodiments 2 and 3, shownin FIGS. 2 and 3, have simpler circular coils and provide betterpossibilities for coil guidance. The choice of planar or cylindricalconfiguration will depend upon the particular application and thetrade-off between these considerations.

FIGS. 4(a) and 4(b) show exemplary dimensions (in mm) for devices of thesecond and third embodiment of this invention. Finite element analysiscalculations have been performed to calculate the performance of deviceswith these dimensions. It is apparent that all the devices areaxi-symmetric, which simplifies the calculations. A number of variationson the type I and type II configurations were considered. FIGS. 5(a) to5(g) show type I devices and FIGS. 6(a) and 6(b) show type II devices.Each Figure shows on the left the configuration of the components of thedevice and on the right a plot of results which will be explained below.The device illustrated in FIG. 5(a) corresponds to the second embodimentof the invention as shown in FIGS. 2 and 4(a). The device in FIG. 6(a)corresponds to the third embodiment of the invention shown in FIGS. 3and 4(b). The dimensions of the components in the further embodimentscan be derived from the fact that FIGS. 5(b) to 5(g) are drawn to thesame scale as FIG. 5(a), and FIG. 6(b) is to the same scale as FIG.6(a).

For each embodiment, the force on the carrier member 20 (coils 26 plusauxiliary magnetic member 28) has been calculated using a Maxwell stressline integral. The coils are taken to have a current density of 10A/mm², and the Lorentz force is calculated by integrating J×B over thevolume of the coils. With the carrier member 20 in its central nominalposition, the total force (F_(T)), Lorentz force (F_(L)) and auxiliaryforce (F_(A), i.e. the bias force due to the auxiliary magnetic member28) are given in the table in FIG. 14; all force values are in N. Thecolumn with parameter T indicates whether the tested device was type Ior II.

Further calculations of the total axial force were performed fordisplacements of the carrier member 20 by 0.5 and 1.0 mm in both the upand down axial directions. A consistent convention was used, such thatpositive force and positive displacement are in the same direction. Allforces and displacements are, of course, essentially relative forces anddisplacements between the carrier member 20 and the magnet yoke 10.

The graphs accompanying FIGS. 5 and 6 plot relative axial displacementin meters along the X axis and total axial force in N along the Y axis.The total axial force for displacements of −1.0, −0.5, 0, 0.5 and 1.0 mmare plotted as the five circles. The dotted line in each graph is acalculated fit to these points. Devices of type I have aforce-versus-displacement curve of at least second order. Devices oftype II have a first order (linear) force-versus-displacement function.

In each plot, the solid line shows the gradient of theforce-versus-displacement plot at the zero displacement, nominalposition. The gradient of this line gives the effective stiffness of thedevice, i.e. the differential change in force with displacement. As canbe seen in FIGS. 5(a) to 5(g), the gradient for type I devices isnegative; thus these have a mechanical spring-like stiffness, in that adisplacement in one direction produces a change in force in the oppositedirection. In contrast, type II actuators have a positive gradient, sodo not behave like a mechanical spring, and have an equivalent stiffnesswhich is negative.

The axial effective stiffness values S are given (in kN/m) in the finalcolumn in FIG. 14. The fact that type II devices have a negativestiffness means that, when the carrier member is displaced in the upwardaxial direction relative to the magnet yoke, the upward force on thecarrier member 20 increases. This means that, when the bias force(auxiliary force) is compensating the weight of some apparatus, there isno stable equilibrium position. However, this is not a problem, becausethe coil currents can be controlled to counteract this negativestiffness as necessary.

As can be derived from the table in FIG. 14, stiffnesses of 1000 N/m orless can be achieved with these devices, and in some cases stiffnessesof less than 200 N/m are possible. The bias force and stiffness can bothbe tuned by choosing the axial dimension of the auxiliary magneticmember and its radial position.

Applications of these devices include use as short stroke drives forreticle and wafer stages in lithographic projection apparatus, and alsoas active bearing systems. The devices according to this invention havethe significant advantage that they can support the weight of a load asan isolation bearing, without heat dissipation due to a base current.Current can, however, still be applied for control and adjustment, or tosupport further loads. The results in FIG. 14 show that examples ofthese devices can provide a bias force at least in the range of 40 to100 N. In a typical lithographic projection apparatus, the moving massof the wafer stage may be of the order of about 15 kg; in such a case,three actuators can (for example) be used, each compensating about 50 Nof weight (i.e. 5 kg).

Another remarkable property of these devices is that, even whenproviding a compensating bias force of, for example, 40 to 100 N, theyhave effective stiffnesses of 1000 N/m or less, and even in some casesstiffnesses below 200 N/m. This would be very difficult to achieve withequivalent mechanical actuators or transducers. These devicesessentially constitute a magnetic bearing with a very large compliance(small stiffness).

Further application of these devices, apart from as Lorentz actuators,is as a velocity transducer. Velocity differences between the coilcarrier member 20 and the magnet yoke 10 a, 10 b cause generation of aproportional EMF in the coil. The bias force due to the auxiliarymagnetic member is used to compensate the weight of the coil carriermember 20 with a very small equivalent stiffness. This gives thetransducer a very low frequency of resonance, and enables it to be usedto measure velocities down to very low frequencies.

Embodiment 5

FIG. 15 schematically depicts a lithographic projection apparatusaccording to the invention. The apparatus comprises:

a radiation system LA, Ex, IN, CO for supplying a projection beam PB ofradiation;

a mask table MT provided with a mask holder for holding a mask MA (e.g.a reticle);

a substrate table WT provided with a substrate holder for holding asubstrate W (e.g. a resist-coated silicon wafer);

a projection system PL (e.g. a lens or catadioptric system, a mirrorgroup or a set of electromagnetic deflectors) for imaging an irradiatedportion of the mask MA onto a target portion C (die) of the substrate W.

As here depicted, the apparatus is transmissive; however, an apparatusemploying reflective components can also be envisioned (e.g. as in thecase of an EUV apparatus).

The radiation system comprises a source LA (e.g. a Hg lamp or excimerlaser, an electron or ion source, or a beam wiggler located around thepath of a particle beam produced by an accelerator) which produces abeam of radiation. This beam is passed along various opticalcomponents,—e.g. beam shaping optics Ex, an integrator IN and acondensor CO—so that the resultant beam PB is substantially collimatedand uniformly intense throughout its cross-section.

The beam PB subsequently intercepts the mask MA which is held in a maskholder on a mask table MT. Having passed through (or been reflectedfrom) the mask MA, the beam PB passes through the projection system PL,which focuses the beam PB onto a target area C of the substrate W. Withthe aid of the interferometric displacement and measuring means IF, thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target areas C in the path of the beam PB.

The depicted apparatus can be used in two different modes:

In step mode, the mask table MT is fixed, and an entire mask image isprojected in one go (i.e. a single “flash”) onto a target area C. Thesubstrate table WT is then shifted in the x and/or y directions so thata different target area C can be irradiated by the (stationary) beam PB;

In scan mode, essentially the same scenario applies, except that a giventarget area C is not exposed in a single “flash”. Instead, the masktable MT is movable in a given direction (the so-called “scandirection”, e.g. the x direction) with a speed v, so that the projectionbeam PB is caused to scan over a mask image; concurrently, the substratetable WT is simultaneously moved in the same or opposite direction at aspeed V=Mv, in which M is the magnification of the projection system PL(typically, M=¼ or ⅕). In this manner, a relatively large target area Ccan be exposed, without having to compromise on resolution.

To achieve the required accuracy, and to avoid excessive wear, the masktable MT and substrate table WT are generally positioned in at least onedegree of freedom (and generally as many as six degrees of freedom) withthe aid of Lorentz actuators. In the Z-direction (i.e. perpendicular tothe surface of the substrate table WT), not only will such a Lorentzactuator be necessary to produce Z-actuations, but some means must alsobe contrived to support the weight of the table being actuated. Theactuator according to the current invention allows such support to beachieved without having to incur significant heat dissipation, and isthus highly advantageous in this application.

Whilst specific embodiments of the invention have been described above,it will be appreciated that the invention may be practised otherwisethan as described.

The Claims accompanying this application have been provided withreference numerals between parentheses. These reference numerals areprovided solely for the purpose of aiding the reader, so as to increaseintelligibility; they should not be construed as limiting the scope ofthe Claims in any way to the particular examples depicted in theFigures.

I claim:
 1. A device comprising: a first member comprising at least onemain magnet; and a second member comprising at least one current elementconstructed and arranged to carry an electric current, toelectromagnetically interact with said main magnet, wherein said secondmember further comprises an auxiliary magnetic member which interactswith a magnetic field of said main magnet to produce a bias forcebetween said first and second members, and wherein said magnet ismagnetized in a direction substantially parallel to said bias force. 2.A device according to claim 1, wherein said first and second members aresubstantially planar.
 3. A device according to claim 1, wherein saidfirst and second members are substantially cylindrical or tubular.
 4. Adevice according to claim 1, further comprising a third membercomprising at least one further magnet.
 5. A device according to claim1, wherein said current element is a coil.
 6. A device according toclaim 5, wherein said auxiliary member is located at a plane which liessubstantially centrally between two halves of said coil.
 7. A deviceaccording to claim 1, wherein the effective stiffness of the device is amaximum of 200 N/m in magnitude.
 8. A device according to claim 1,wherein said device is an actuator in which passage of current throughsaid current element induces a controllable relative force between saidfirst and second members.
 9. A device according to claim 1, wherein saiddevice is a transducer in which a velocity difference between said firstand second members can be sensed by way of an EMF induced in saidcurrent element.
 10. A device according to claim 1, comprising twosecond members stiffly connected to each other and arranged such that,in use, opposite parasitic torques are generated in each second member,which thereby cancel out.
 11. A lithographic projection apparatuscomprising: a mask table provided with a mask holder constructed andarranged to hold a mask; a substrate table provided with a substrateholder constructed and arranged to hold a substrate; a projection systemconstructed and arranged to image an irradiated portion of the mask ontoa target portions of the substrate, wherein the apparatus furthercomprises a device comprising: a first member comprising at least onemain magnet; and a second member comprising at least one current elementconstructed and arranged to carry an electric current, toelectromagnetically interact with said main magnet, wherein said secondmember further comprises an auxiliary magnetic member which interactswith a magnetic field of said main magnet to produce a bias forcebetween said first and second members, and wherein said main magnet ismagnetized in a direction substantially parallel to said bias force. 12.An apparatus according to claim 11, wherein said first and secondmembers are substantially planar.
 13. An apparatus according to claim11, wherein said first and second members are substantially cylindricalor tubular.
 14. An apparatus according to claim 11, further comprising athird member comprising at least one further main magnet.
 15. Anapparatus according to claim 11, wherein said current element is a coil.16. An apparatus according to claim 15, wherein said auxiliary magneticmember is located at a plane which lies substantially centrally betweentwo halves of said coil.
 17. An apparatus according to claim 11, whereinthe effective stiffness of the device is 200 N/m or less in magnitude.18. An apparatus according to claim 11, wherein said device is anactuator in which passage of current through said current elementinduces a controllable relative force between said first and secondmembers.
 19. An apparatus according to claim 11, wherein said device isa transducer in which a velocity difference between said first andsecond members can be sensed by way of an EMF induced in said currentelement.
 20. An apparatus according to claim 11, comprising two secondmembers stiffly connected to each other and arranged such that, in use,opposite parasitic torques are generated in each second member whichthereby cancel out.